There is a small race between do_fork() and sched_move_task(), which is
trying to move the child.
do_fork() sched_move_task()
--------------------------------+---------------------------------
copy_process()
sched_fork()
task_fork_fair()
-> vruntime of the child is initialized
based on that of the parent.
-> we can see the child in "tasks" file now.
task_rq_lock()
task_move_group_fair()
-> child.se.vruntime
-= (old)cfs_rq->min_vruntime
+= (new)cfs_rq->min_vruntime
task_rq_unlock()
wake_up_new_task()
...
enqueue_entity()
child.se.vruntime += cfs_rq->min_vruntime
As a result, vruntime of the child becomes far bigger than min_vruntime,
if (new)cfs_rq->min_vruntime >> (old)cfs_rq->min_vruntime.
This patch fixes this problem by just ignoring such process in
task_move_group_fair(), because the vruntime has already been normalized in
task_fork_fair().
Signed-off-by: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
---
kernel/sched/fair.c | 13 +++++++++++++
1 files changed, 13 insertions(+), 0 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a4d2b7a..43cadd0 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5336,6 +5336,19 @@ static void task_move_group_fair(struct task_struct *p, int on_rq)
* to another cgroup's rq. This does somewhat interfere with the
* fair sleeper stuff for the first placement, but who cares.
*/
+ /*
+ * When !on_rq, vruntime of the task has usually NOT been normalized.
+ * But there are some cases where it has already been normalized:
+ *
+ * - Moving a forked child which is waiting for being woken up by
+ * wake_up_new_task().
+ *
+ * To prevent boost or penalty in the new cfs_rq caused by delta
+ * min_vruntime between the two cfs_rqs, we skip vruntime adjustment.
+ */
+ if (!on_rq && !p->se.sum_exec_runtime)
+ on_rq = 1;
+
if (!on_rq)
p->se.vruntime -= cfs_rq_of(&p->se)->min_vruntime;
set_task_rq(p, task_cpu(p));
--
1.7.1
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